Untitled DocumentGeochemical identification of riparian water sources
Macro Theme Area:
River Systems [Project ID: R01]
PI:
James Hogan
CO-PI(s):
N/A
Basin focus:
San Pedro
Specific area in
basin /
field sites:
Upper San Pedro River Basin
Summary/Goals: Water managers of groundwater basins in the semiarid Southwest U.S. must balance the demands of growing populations against the needs of riparian ecosystems. The Upper San Pedro River Basin in Southeastern Arizona supports one of the few remaining riparian areas in this region by providing baseflow to the San Pedro River and shallow groundwater to phreatic riparian vegetation. The aquifer in this basin also provides water for the rapidly growing area of Fort Huachuca, Sierra Vista, and surrounding communities. Before water managers can properly plan for sustenance of the riparian area, they require answers about the inputs of baseflow into the river. What is the temporal and spatial variability of recharge and discharge? What are the relative inputs of monsoon runoff and basin groundwater into the riparian aquifer? This project seeks to quantify inputs to flow in the San Pedro River from different sources, including mountain block recharge, mountain front recharge, ephemeral channel recharge, and riparian monsoon flow recharge. To do so, we analyzed a suite of geochemical tracers present in the groundwater of the area. The constituents we used include stable isotopes, radioactive isotopes, major anions, and noble gases.
Activities and outcomes during past year:
The Upper San Pedro River Basin (Arizona, U.S.A.) contains one of the few remaining desert riparian areas in the Southwest. This study characterizes and quantifies the water sources that contribute to riparian groundwater and sustain river baseflow using a suite of geochemical tracers including stable and radioactive isotopes and anions. Most precipitation in this region falls during the summer monsoons and winter frontal storms. Because these events have different source areas, their respective stable isotopic signatures differ, allowing us to determine the dominant season for recharge. Groundwater in the basin has a narrow range of stable isotope compositions, varying from 62 to 72% winter precipitation. The basin isotopic composition is very similar to water in the mountain front and block, indicating that recharge from these areas supplies basin groundwater. Basin groundwater residence times, determined using the radioactive isotope carbon-14, are on the order of 10,000 years or more, indicating a low recharge rate. Results indicate that of the possible sources, basin groundwater from the west and local recharge of monsoon floodwaters are the most significant water sources to the riparian system. Using a simple two end-member mixing model, riparian groundwater varies from 10 to 90% basin groundwater whereas baseflow ranges from 0 to 55%; the percentage is well correlated with gaining (basin groundwater dominated) and losing reaches (monsoon dominated). Tritium age data indicate that some riparian groundwater, especially in losing reaches, has been recharged recently (less than 40 years). Baseflow varies from 40 to 100% monsoon precipitation, and is closer to basin groundwater in gaining reaches. These results help can water managers better understand the water resources of the basin, and increase their ability to effectively plan for the future health of the riparian area. For example, the demonstrated importance of monsoon recharge to the riparian aquifer indicates that effective strategies could include construction of retention basins to capture monsoon runoff for recharge.
Plans for the upcoming year:
For 2006 our focus will be on publication of the results. To date our results have only been published as abstracts and as a MS Thesis. We will search for an appropriate peer-reviewed journal in which to publish.
Organization Involvement:
We arranged to make a presentation to the Technical Committee of the Upper San Pedro Partnership (USPP). This was done for two reasons. First, as the major stakeholder group in the basin we wanted to take the opportunity to present our research plan and gather their advice as to how the project could be improved or modified to meet their management needs as well as our research needs. The second reason was to obtain the permission and contacts needed for well sampling. The presentation was well received and the members of the technical committee thought the research had the possibility of
greatly increasing their understanding of the hydrology of the Huachuca Mountains.
Shared Resources / Joint Activities:
The committee indicated a willingness to fund additional research on this topic if
the initial results generated from TRIF funding proved promising.
Organization Involvement:
In support of outreach efforts associated with this project, we traveled to Kartchner
Caverns in early June to meet with State Park officials and other TRIF funded
researchers to discuss the development of an interactive display on hydrologic
literacy to be housed in the Kartchner Caverns Discovery Center. Researchers
associated with this project agreed to develop content associated with Cave
Hydrology, Mountain Block and Mountain Front Recharge, Isotope Dating of Cave
formations and Groundwater Ages and Basin Hydrology (linking the cave and
mountains to the San Pedro River).
Shared Resources / Joint Activities:
Exhibit space at the Kartchner Caverns Discovery Center.
Organization Involvement:
Development of Kartchner Caverns display on "Cave Hydrology"
Shared Resources / Joint Activities:
N/A
Location: Wilcox, AZ, USA
URL: N/A
United States Geological Survey
Organization Involvement:
We met with USGS researchers working in the San Pedro basin. This was done to discuss how our results might assist current USGS activities and to garner advice on well sampling locations. In particular we focused on how our age dating information might be used to calibrate the latest USGS groundwater model for the basin.
